Syntheses of oxygen-18 and deuterium labeled compounds are being developed for clinical studies on their metabolic fate and for internal standards in mass spectral assays. Synthesis of dioxygen-18 labeled 3,4 dihydroxyphenylethyleneglycol (DHPG) has been completed in quantities sufficient for animal and human studies. Analogs of melatonin and its metabolites are being synthesized.